216 FOUNDERS OF OCEANOGRAPHY 



was probably caused by a vast swarm of Medusae excited 

 to luminescence by either an internal periodic or an external 

 accidental stimulation, such as a passing fish or a colUsion 

 of two or more of the Medusae. The stimulation of one of 

 the crowd might be sufficient to start them all. The appear- 

 ance from the deck of our ship was as if first one of the globes 

 lit up and then another and another in rapid succession, 

 suggesting that the luminescence of the one was stimulating 

 the others to similar action. 



The most brilliant light-producing Medusa in our own 

 seas is Pelagia noctiluca. A small tankful of them once 

 gave us a magnificent display in the dark at the Port Erin 

 Biological Station, and when taken out in a bucket they 

 looked like balls of fire, or rather incandescent metal, as 

 the light is white and very intense. It was difficult to 

 believe it would not burn one's fingers when touched. 



Alexander Agassiz has recorded that in the luminous 

 Ctenophora (such as Pleurobrachia, Plate XIII, Fig. 2), not 

 only the adults but even young embryos are luminous, which 

 shows that the light -producing material is not necessarily 

 the secretion of a special gland, but may be formed in the 

 protoplasm of the early cells. 



The colonial Coelenterates, when luminescent, remind one 

 of fireworks or electric -light displays, as all the polypes, or 

 groups of poljrpes, glow out one after another till the whole 

 series of branches are ablaze. It is impossible to resist the 

 conclusion that the stimulus spreads from one member of 

 the colony to another. This is typically the case in the 

 well-known sea-pen Pennatula phosjjhorea, so named by 

 Linnaeus in the eighteenth century, but known as a luminous 

 animal by Gesner a couple of centuries before, and probably 

 by others still earlier. (See Plate XVI, Fig. 1.) 



This, like Noctiluca, is a classical example of luminescence 

 amongst British animals ; and when taken into a dark 

 cabin immediately on being brought up in the dredge, 

 Pennatula fhosphorea is a wonderfully beautiful sight. The 



